This invention relates to a method of fabricating a polycrystalline silicon wafer used for a solar battery, a photoelectric converter, etc. and a fabrication tray capable of executing the method.
A polycrystalline silicon wafer is heretofore fabricated by a variety of methods. Generally, an ingot of prescribed shape is temporarily cast from a silicon base material, and is then sliced to fabricate a wafer. Since this conventional method takes a long time to slice the ingot and yet approx. 50% of the ingot is wasted at the time of slicing the ingot, the product of the wafer becomes expensive, and it is further impossible to fabricate a number of wafers in a mass production.
A ribbon method and a casting method are already executed as a method without slicing step for that purpose. The ribbon method has, for example, the steps of injecting molten silicon to the peripheral surface of a rotating drum and forming a ribbon-shaped wafer on the peripheral surface of the drum. According to this ribbon method, the ribbon having a width of several millimeters can only be fabricated, and it has such difficulty that a solar battery cell of large size cannot be fabricated.
The casting method has the steps of heating a silicon base material into its molten liquid, pouring the molten silicon material in a mold prepared in response to the size of the wafer of the product, and further press molding the molten material by the movable mold to solidify the molten material. According to this casting method, the wafer of prescribed shape can be obtained simultaneously and a preferred result can be expected at the viewpoint of its mass production, but the molten silicon base material is urged from all the peripheral directions.
Thus, the growth of silicon crystalline grains is disadvantageously suppressed according to this casting method when the molten silicon material is solidified among the upper, lower and side surfaces of the casting mold. This causes the vicinity of the parts of the silicon material to be solidified in contact with the upper, lower and side surfaces of the mold to become extremely fine crystalline grains, but cannot obtain large crystalline grains. This does not satisfy the requirements of the fabrication of large crystalline grains which are desired for a silicon wafer used for a solar battery cell. Accordingly, the photoelectric conversion efficiency of the solar battery thus obtained with the wafer is remarkably deteriorated to 2 to 3% as its drawbacks and disadvantages.
The inventors of the present invention have already proposed, as a method of fabricating a polycrystalline silicon wafer capable of largely improving the disadvantages of the above-described conventional methods, a method (a spinning method) which has the steps of melting a silicon base material, dropping the molten material on a rotating fabrication tray formed of quartz or carbon, forming a thin molten material layer of desirably increased diameter by utilizing centrifugal force, solidifying the molten material layer, and then isolating the silicon sheet from the tray.
This spinning method has a number of excellent features, but when the thin molten material layer is solidified, the expanded portion of the molten material produced when the molten material is solidified at high temperature affects the volumetric expansion which occurs at that time relative to the free surface of the molten material which was originally smooth, and small projections are disadvantageously formed in groups on the surface of the silicon sheet of the resultant product (on the opposite to the bonded surface which is surface-contacted with the surface of the tray).
In the conventional method, the small projections should be removed by various etchants and the surface of the silicon sheet should be smoothly finished by manual works. This work is extremely complicated, and when the height of the projections is higher than 0.5 mm, the removal of etchants becomes impossible, resulting in the large decrease in the production efficiency.